1. Technical Field
The present invention generally relates to methods and apparatus for adapting transmit parameters to radio conditions in a wireless communications system, and particularly relates to adapting mobile terminal transmit parameters, including the bandwidth of the transmitted signal, in order to reduce the energy consumption and extend the battery life of the mobile terminal.
2. Background
The 3rd Generation Partnership Project (3GPP) is currently developing specifications for new wireless communications systems as part of its “Long Term Evolution” (LTE) initiative. The goals of LTE include very high peak data rates (up to 100 Mbps on the downlink; up to 50 Mbps on the uplink) for mobile users. In order to achieve these goals, LTE as currently planned employs advanced multiple access schemes, adaptive modulation and coding schemes, and advanced multi-antenna technologies.
In particular, 3GPP has selected single carrier frequency division multiple access (SC-FDMA) technology for the LTE uplink (transmissions from a mobile terminal to a base terminal). Although similar in many respects to orthogonal frequency division multiple access (OFDMA) technology, SC-FDMA signals offer a reduced peak-to-average power ratio (PAPR) compared to OFDMA signals, thus allowing transmitter power amplifiers to be operated more efficiently. This in turn facilitates more efficient usage of a mobile terminal's limited battery resources. (SC-FDMA is described more fully in Myung, et al., “Single Carrier FDMA for Uplink Wireless Transmission,” IEEE Vehicular Technology Magazine, vol. 1, no. 3, Sep. 2006, pp. 30-38.)
LTE resource blocks are defined as time-frequency blocks with a duration of 0.5 milliseconds (one slot, or half a sub-frame) and encompassing a bandwidth of 180 kHz (corresponding to 12 sub-carriers with a spacing of 15 kHz). The exact definition, for example the duration and bandwidth of a resource block, may vary within LTE and similar systems, and the invention is not limited to the numbers used herein. Resource blocks may be dynamically assigned to mobile terminals, and may be assigned independently for the uplink (reverse link) and the downlink (forward link). Depending on a mobile terminal's requirements, the system resources allocated to it may be increased by allocating resource blocks across several sub-frames, or across several frequency blocks, or both. Thus, the instantaneous bandwidth allocated to a mobile terminal transmitter in a scheduling process may be dynamically adapted to respond to changing conditions.
In addition, LTE also employs multiple modulation formats (including at least QPSK, 16-QAM, and 64-QAM), as well as advanced coding techniques, so that maximum throughput can be achieved over a variety of signal conditions. Depending on the signal conditions and the desired data rate, a suitable combination of modulation format, coding scheme, and bandwidth is chosen, generally to maximize the system throughput. Power control is also employed to ensure acceptable bit error rates while minimizing interference between cells.
Although SC-FDMA was selected as the LTE uplink multiple access scheme at least partly to facilitate more efficient use of mobile terminal battery resources, the allocation of link resources and selection of transmit parameters such as modulation format, coding scheme, and transmit power level is generally performed so as to maximize the overall system throughput, perhaps while maintaining a certain quality of service (QoS) for each mobile terminal. Maximum system throughput, however, will not generally coincide with maximum efficiency (i.e. lowest overall energy consumption) at the mobile terminal. Past experience has shown that lower energy consumption, which translates directly into longer battery life between charges, is a key criterion in ensuring consumer satisfaction with mobile devices.